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                    竞争条件检测
            
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        </li>
    
        <li class="chapter " data-level="1.10.7" data-path="../ch9/ch9-07.html">
            
                <a href="../ch9/ch9-07.html">
            
                    
                    示例: 并发的非阻塞缓存
            
                </a>
            

            
        </li>
    
        <li class="chapter " data-level="1.10.8" data-path="../ch9/ch9-08.html">
            
                <a href="../ch9/ch9-08.html">
            
                    
                    Goroutines和线程
            
                </a>
            

            
        </li>
    

            </ul>
            
        </li>
    
        <li class="chapter " data-level="1.11" data-path="../ch10/ch10.html">
            
                <a href="../ch10/ch10.html">
            
                    
                    包和工具
            
                </a>
            

            
            <ul class="articles">
                
    
        <li class="chapter " data-level="1.11.1" data-path="../ch10/ch10-01.html">
            
                <a href="../ch10/ch10-01.html">
            
                    
                    包简介
            
                </a>
            

            
        </li>
    
        <li class="chapter " data-level="1.11.2" data-path="../ch10/ch10-02.html">
            
                <a href="../ch10/ch10-02.html">
            
                    
                    导入路径
            
                </a>
            

            
        </li>
    
        <li class="chapter " data-level="1.11.3" data-path="../ch10/ch10-03.html">
            
                <a href="../ch10/ch10-03.html">
            
                    
                    包声明
            
                </a>
            

            
        </li>
    
        <li class="chapter " data-level="1.11.4" data-path="../ch10/ch10-04.html">
            
                <a href="../ch10/ch10-04.html">
            
                    
                    导入声明
            
                </a>
            

            
        </li>
    
        <li class="chapter " data-level="1.11.5" data-path="../ch10/ch10-05.html">
            
                <a href="../ch10/ch10-05.html">
            
                    
                    包的匿名导入
            
                </a>
            

            
        </li>
    
        <li class="chapter " data-level="1.11.6" data-path="../ch10/ch10-06.html">
            
                <a href="../ch10/ch10-06.html">
            
                    
                    包和命名
            
                </a>
            

            
        </li>
    
        <li class="chapter " data-level="1.11.7" data-path="../ch10/ch10-07.html">
            
                <a href="../ch10/ch10-07.html">
            
                    
                    工具
            
                </a>
            

            
        </li>
    

            </ul>
            
        </li>
    
        <li class="chapter " data-level="1.12" data-path="../ch11/ch11.html">
            
                <a href="../ch11/ch11.html">
            
                    
                    测试
            
                </a>
            

            
            <ul class="articles">
                
    
        <li class="chapter " data-level="1.12.1" data-path="../ch11/ch11-01.html">
            
                <a href="../ch11/ch11-01.html">
            
                    
                    go test
            
                </a>
            

            
        </li>
    
        <li class="chapter " data-level="1.12.2" data-path="../ch11/ch11-02.html">
            
                <a href="../ch11/ch11-02.html">
            
                    
                    测试函数
            
                </a>
            

            
        </li>
    
        <li class="chapter " data-level="1.12.3" data-path="../ch11/ch11-03.html">
            
                <a href="../ch11/ch11-03.html">
            
                    
                    测试覆盖率
            
                </a>
            

            
        </li>
    
        <li class="chapter " data-level="1.12.4" data-path="../ch11/ch11-04.html">
            
                <a href="../ch11/ch11-04.html">
            
                    
                    基准测试
            
                </a>
            

            
        </li>
    
        <li class="chapter " data-level="1.12.5" data-path="../ch11/ch11-05.html">
            
                <a href="../ch11/ch11-05.html">
            
                    
                    剖析
            
                </a>
            

            
        </li>
    
        <li class="chapter " data-level="1.12.6" data-path="../ch11/ch11-06.html">
            
                <a href="../ch11/ch11-06.html">
            
                    
                    示例函数
            
                </a>
            

            
        </li>
    

            </ul>
            
        </li>
    
        <li class="chapter " data-level="1.13" data-path="../ch12/ch12.html">
            
                <a href="../ch12/ch12.html">
            
                    
                    反射
            
                </a>
            

            
            <ul class="articles">
                
    
        <li class="chapter " data-level="1.13.1" data-path="../ch12/ch12-01.html">
            
                <a href="../ch12/ch12-01.html">
            
                    
                    为何需要反射?
            
                </a>
            

            
        </li>
    
        <li class="chapter " data-level="1.13.2" data-path="../ch12/ch12-02.html">
            
                <a href="../ch12/ch12-02.html">
            
                    
                    reflect.Type和reflect.Value
            
                </a>
            

            
        </li>
    
        <li class="chapter " data-level="1.13.3" data-path="../ch12/ch12-03.html">
            
                <a href="../ch12/ch12-03.html">
            
                    
                    Display递归打印
            
                </a>
            

            
        </li>
    
        <li class="chapter " data-level="1.13.4" data-path="../ch12/ch12-04.html">
            
                <a href="../ch12/ch12-04.html">
            
                    
                    示例: 编码S表达式
            
                </a>
            

            
        </li>
    
        <li class="chapter " data-level="1.13.5" data-path="../ch12/ch12-05.html">
            
                <a href="../ch12/ch12-05.html">
            
                    
                    通过reflect.Value修改值
            
                </a>
            

            
        </li>
    
        <li class="chapter " data-level="1.13.6" data-path="../ch12/ch12-06.html">
            
                <a href="../ch12/ch12-06.html">
            
                    
                    示例: 解码S表达式
            
                </a>
            

            
        </li>
    
        <li class="chapter " data-level="1.13.7" data-path="../ch12/ch12-07.html">
            
                <a href="../ch12/ch12-07.html">
            
                    
                    获取结构体字段标签
            
                </a>
            

            
        </li>
    
        <li class="chapter " data-level="1.13.8" data-path="../ch12/ch12-08.html">
            
                <a href="../ch12/ch12-08.html">
            
                    
                    显示一个类型的方法集
            
                </a>
            

            
        </li>
    
        <li class="chapter " data-level="1.13.9" data-path="../ch12/ch12-09.html">
            
                <a href="../ch12/ch12-09.html">
            
                    
                    几点忠告
            
                </a>
            

            
        </li>
    

            </ul>
            
        </li>
    
        <li class="chapter " data-level="1.14" data-path="../ch13/ch13.html">
            
                <a href="../ch13/ch13.html">
            
                    
                    底层编程
            
                </a>
            

            
            <ul class="articles">
                
    
        <li class="chapter " data-level="1.14.1" data-path="../ch13/ch13-01.html">
            
                <a href="../ch13/ch13-01.html">
            
                    
                    unsafe.Sizeof, Alignof 和 Offsetof
            
                </a>
            

            
        </li>
    
        <li class="chapter " data-level="1.14.2" data-path="../ch13/ch13-02.html">
            
                <a href="../ch13/ch13-02.html">
            
                    
                    unsafe.Pointer
            
                </a>
            

            
        </li>
    
        <li class="chapter " data-level="1.14.3" data-path="../ch13/ch13-03.html">
            
                <a href="../ch13/ch13-03.html">
            
                    
                    示例: 深度相等判断
            
                </a>
            

            
        </li>
    
        <li class="chapter " data-level="1.14.4" data-path="../ch13/ch13-04.html">
            
                <a href="../ch13/ch13-04.html">
            
                    
                    通过cgo调用C代码
            
                </a>
            

            
        </li>
    
        <li class="chapter " data-level="1.14.5" data-path="../ch13/ch13-05.html">
            
                <a href="../ch13/ch13-05.html">
            
                    
                    几点忠告
            
                </a>
            

            
        </li>
    

            </ul>
            
        </li>
    
        <li class="chapter " data-level="1.15" data-path="../appendix/appendix.html">
            
                <a href="../appendix/appendix.html">
            
                    
                    附录
            
                </a>
            

            
            <ul class="articles">
                
    
        <li class="chapter " data-level="1.15.1" data-path="../appendix/appendix-a-errata.html">
            
                <a href="../appendix/appendix-a-errata.html">
            
                    
                    附录A：原文勘误
            
                </a>
            

            
        </li>
    
        <li class="chapter " data-level="1.15.2" data-path="../appendix/appendix-b-author.html">
            
                <a href="../appendix/appendix-b-author.html">
            
                    
                    附录B：作者译者
            
                </a>
            

            
        </li>
    
        <li class="chapter " data-level="1.15.3" data-path="../appendix/appendix-c-cpoyright.html">
            
                <a href="../appendix/appendix-c-cpoyright.html">
            
                    
                    附录C：译文授权
            
                </a>
            

            
        </li>
    
        <li class="chapter " data-level="1.15.4" data-path="../appendix/appendix-d-translations.html">
            
                <a href="../appendix/appendix-d-translations.html">
            
                    
                    附录D：其它语言
            
                </a>
            

            
        </li>
    

            </ul>
            
        </li>
    

    

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                                <h2 id="58-deferred&#x51FD;&#x6570;">5.8. Deferred&#x51FD;&#x6570;</h2>
<p>&#x5728;findLinks&#x7684;&#x4F8B;&#x5B50;&#x4E2D;&#xFF0C;&#x6211;&#x4EEC;&#x7528;http.Get&#x7684;&#x8F93;&#x51FA;&#x4F5C;&#x4E3A;html.Parse&#x7684;&#x8F93;&#x5165;&#x3002;&#x53EA;&#x6709;url&#x7684;&#x5185;&#x5BB9;&#x7684;&#x786E;&#x662F;HTML&#x683C;&#x5F0F;&#x7684;&#xFF0C;html.Parse&#x624D;&#x53EF;&#x4EE5;&#x6B63;&#x5E38;&#x5DE5;&#x4F5C;&#xFF0C;&#x4F46;&#x5B9E;&#x9645;&#x4E0A;&#xFF0C;url&#x6307;&#x5411;&#x7684;&#x5185;&#x5BB9;&#x5F88;&#x4E30;&#x5BCC;&#xFF0C;&#x53EF;&#x80FD;&#x662F;&#x56FE;&#x7247;&#xFF0C;&#x7EAF;&#x6587;&#x672C;&#x6216;&#x662F;&#x5176;&#x4ED6;&#x3002;&#x5C06;&#x8FD9;&#x4E9B;&#x683C;&#x5F0F;&#x7684;&#x5185;&#x5BB9;&#x4F20;&#x9012;&#x7ED9;html.parse&#xFF0C;&#x4F1A;&#x4EA7;&#x751F;&#x4E0D;&#x826F;&#x540E;&#x679C;&#x3002;</p>
<p>&#x4E0B;&#x9762;&#x7684;&#x4F8B;&#x5B50;&#x83B7;&#x53D6;HTML&#x9875;&#x9762;&#x5E76;&#x8F93;&#x51FA;&#x9875;&#x9762;&#x7684;&#x6807;&#x9898;&#x3002;title&#x51FD;&#x6570;&#x4F1A;&#x68C0;&#x67E5;&#x670D;&#x52A1;&#x5668;&#x8FD4;&#x56DE;&#x7684;Content-Type&#x5B57;&#x6BB5;&#xFF0C;&#x5982;&#x679C;&#x53D1;&#x73B0;&#x9875;&#x9762;&#x4E0D;&#x662F;HTML&#xFF0C;&#x5C06;&#x7EC8;&#x6B62;&#x51FD;&#x6570;&#x8FD0;&#x884C;&#xFF0C;&#x8FD4;&#x56DE;&#x9519;&#x8BEF;&#x3002;</p>
<p><u><i>gopl.io/ch5/title1</i></u></p>
<pre><code class="lang-Go"><span class="hljs-keyword">func</span> title(url <span class="hljs-keyword">string</span>) error {
    resp, err := http.Get(url)
    <span class="hljs-keyword">if</span> err != <span class="hljs-literal">nil</span> {
        <span class="hljs-keyword">return</span> err
    }
    <span class="hljs-comment">// Check Content-Type is HTML (e.g., &quot;text/html;charset=utf-8&quot;).</span>
    ct := resp.Header.Get(<span class="hljs-string">&quot;Content-Type&quot;</span>)
    <span class="hljs-keyword">if</span> ct != <span class="hljs-string">&quot;text/html&quot;</span> &amp;&amp; !strings.HasPrefix(ct,<span class="hljs-string">&quot;text/html;&quot;</span>) {
        resp.Body.Close()
        <span class="hljs-keyword">return</span> fmt.Errorf(<span class="hljs-string">&quot;%s has type %s, not text/html&quot;</span>,url, ct)
    }
    doc, err := html.Parse(resp.Body)
    resp.Body.Close()
    <span class="hljs-keyword">if</span> err != <span class="hljs-literal">nil</span> {
        <span class="hljs-keyword">return</span> fmt.Errorf(<span class="hljs-string">&quot;parsing %s as HTML: %v&quot;</span>, url,err)
    }
    visitNode := <span class="hljs-keyword">func</span>(n *html.Node) {
        <span class="hljs-keyword">if</span> n.Type == html.ElementNode &amp;&amp; n.Data == <span class="hljs-string">&quot;title&quot;</span>&amp;&amp;n.FirstChild != <span class="hljs-literal">nil</span> {
            fmt.Println(n.FirstChild.Data)
        }
    }
    forEachNode(doc, visitNode, <span class="hljs-literal">nil</span>)
    <span class="hljs-keyword">return</span> <span class="hljs-literal">nil</span>
}
</code></pre>
<p>&#x4E0B;&#x9762;&#x5C55;&#x793A;&#x4E86;&#x8FD0;&#x884C;&#x6548;&#x679C;&#xFF1A;</p>
<pre><code>$ go build gopl.io/ch5/title1
$ ./title1 http://gopl.io
The Go Programming Language
$ ./title1 https://golang.org/doc/effective_go.html
Effective Go - The Go Programming Language
$ ./title1 https://golang.org/doc/gopher/frontpage.png
title1: https://golang.org/doc/gopher/frontpage.png has type image/png, not text/html
</code></pre><p>resp.Body.close&#x8C03;&#x7528;&#x4E86;&#x591A;&#x6B21;&#xFF0C;&#x8FD9;&#x662F;&#x4E3A;&#x4E86;&#x786E;&#x4FDD;title&#x5728;&#x6240;&#x6709;&#x6267;&#x884C;&#x8DEF;&#x5F84;&#x4E0B;&#xFF08;&#x5373;&#x4F7F;&#x51FD;&#x6570;&#x8FD0;&#x884C;&#x5931;&#x8D25;&#xFF09;&#x90FD;&#x5173;&#x95ED;&#x4E86;&#x7F51;&#x7EDC;&#x8FDE;&#x63A5;&#x3002;&#x968F;&#x7740;&#x51FD;&#x6570;&#x53D8;&#x5F97;&#x590D;&#x6742;&#xFF0C;&#x9700;&#x8981;&#x5904;&#x7406;&#x7684;&#x9519;&#x8BEF;&#x4E5F;&#x53D8;&#x591A;&#xFF0C;&#x7EF4;&#x62A4;&#x6E05;&#x7406;&#x903B;&#x8F91;&#x53D8;&#x5F97;&#x8D8A;&#x6765;&#x8D8A;&#x56F0;&#x96BE;&#x3002;&#x800C;Go&#x8BED;&#x8A00;&#x72EC;&#x6709;&#x7684;defer&#x673A;&#x5236;&#x53EF;&#x4EE5;&#x8BA9;&#x4E8B;&#x60C5;&#x53D8;&#x5F97;&#x7B80;&#x5355;&#x3002;</p>
<p>&#x4F60;&#x53EA;&#x9700;&#x8981;&#x5728;&#x8C03;&#x7528;&#x666E;&#x901A;&#x51FD;&#x6570;&#x6216;&#x65B9;&#x6CD5;&#x524D;&#x52A0;&#x4E0A;&#x5173;&#x952E;&#x5B57;defer&#xFF0C;&#x5C31;&#x5B8C;&#x6210;&#x4E86;defer&#x6240;&#x9700;&#x8981;&#x7684;&#x8BED;&#x6CD5;&#x3002;&#x5F53;&#x6267;&#x884C;&#x5230;&#x8BE5;&#x6761;&#x8BED;&#x53E5;&#x65F6;&#xFF0C;&#x51FD;&#x6570;&#x548C;&#x53C2;&#x6570;&#x8868;&#x8FBE;&#x5F0F;&#x5F97;&#x5230;&#x8BA1;&#x7B97;&#xFF0C;&#x4F46;&#x76F4;&#x5230;&#x5305;&#x542B;&#x8BE5;defer&#x8BED;&#x53E5;&#x7684;&#x51FD;&#x6570;&#x6267;&#x884C;&#x5B8C;&#x6BD5;&#x65F6;&#xFF0C;defer&#x540E;&#x7684;&#x51FD;&#x6570;&#x624D;&#x4F1A;&#x88AB;&#x6267;&#x884C;&#xFF0C;&#x4E0D;&#x8BBA;&#x5305;&#x542B;defer&#x8BED;&#x53E5;&#x7684;&#x51FD;&#x6570;&#x662F;&#x901A;&#x8FC7;return&#x6B63;&#x5E38;&#x7ED3;&#x675F;&#xFF0C;&#x8FD8;&#x662F;&#x7531;&#x4E8E;panic&#x5BFC;&#x81F4;&#x7684;&#x5F02;&#x5E38;&#x7ED3;&#x675F;&#x3002;&#x4F60;&#x53EF;&#x4EE5;&#x5728;&#x4E00;&#x4E2A;&#x51FD;&#x6570;&#x4E2D;&#x6267;&#x884C;&#x591A;&#x6761;defer&#x8BED;&#x53E5;&#xFF0C;&#x5B83;&#x4EEC;&#x7684;&#x6267;&#x884C;&#x987A;&#x5E8F;&#x4E0E;&#x58F0;&#x660E;&#x987A;&#x5E8F;&#x76F8;&#x53CD;&#x3002;</p>
<p>defer&#x8BED;&#x53E5;&#x7ECF;&#x5E38;&#x88AB;&#x7528;&#x4E8E;&#x5904;&#x7406;&#x6210;&#x5BF9;&#x7684;&#x64CD;&#x4F5C;&#xFF0C;&#x5982;&#x6253;&#x5F00;&#x3001;&#x5173;&#x95ED;&#x3001;&#x8FDE;&#x63A5;&#x3001;&#x65AD;&#x5F00;&#x8FDE;&#x63A5;&#x3001;&#x52A0;&#x9501;&#x3001;&#x91CA;&#x653E;&#x9501;&#x3002;&#x901A;&#x8FC7;defer&#x673A;&#x5236;&#xFF0C;&#x4E0D;&#x8BBA;&#x51FD;&#x6570;&#x903B;&#x8F91;&#x591A;&#x590D;&#x6742;&#xFF0C;&#x90FD;&#x80FD;&#x4FDD;&#x8BC1;&#x5728;&#x4EFB;&#x4F55;&#x6267;&#x884C;&#x8DEF;&#x5F84;&#x4E0B;&#xFF0C;&#x8D44;&#x6E90;&#x88AB;&#x91CA;&#x653E;&#x3002;&#x91CA;&#x653E;&#x8D44;&#x6E90;&#x7684;defer&#x5E94;&#x8BE5;&#x76F4;&#x63A5;&#x8DDF;&#x5728;&#x8BF7;&#x6C42;&#x8D44;&#x6E90;&#x7684;&#x8BED;&#x53E5;&#x540E;&#x3002;&#x5728;&#x4E0B;&#x9762;&#x7684;&#x4EE3;&#x7801;&#x4E2D;&#xFF0C;&#x4E00;&#x6761;defer&#x8BED;&#x53E5;&#x66FF;&#x4EE3;&#x4E86;&#x4E4B;&#x524D;&#x7684;&#x6240;&#x6709;resp.Body.Close</p>
<p><u><i>gopl.io/ch5/title2</i></u></p>
<pre><code class="lang-Go"><span class="hljs-keyword">func</span> title(url <span class="hljs-keyword">string</span>) error {
    resp, err := http.Get(url)
    <span class="hljs-keyword">if</span> err != <span class="hljs-literal">nil</span> {
        <span class="hljs-keyword">return</span> err
    }
    <span class="hljs-keyword">defer</span> resp.Body.Close()
    ct := resp.Header.Get(<span class="hljs-string">&quot;Content-Type&quot;</span>)
    <span class="hljs-keyword">if</span> ct != <span class="hljs-string">&quot;text/html&quot;</span> &amp;&amp; !strings.HasPrefix(ct,<span class="hljs-string">&quot;text/html;&quot;</span>) {
        <span class="hljs-keyword">return</span> fmt.Errorf(<span class="hljs-string">&quot;%s has type %s, not text/html&quot;</span>,url, ct)
    }
    doc, err := html.Parse(resp.Body)
    <span class="hljs-keyword">if</span> err != <span class="hljs-literal">nil</span> {
        <span class="hljs-keyword">return</span> fmt.Errorf(<span class="hljs-string">&quot;parsing %s as HTML: %v&quot;</span>, url,err)
    }
    <span class="hljs-comment">// ...print doc&apos;s title element&#x2026;</span>
    <span class="hljs-keyword">return</span> <span class="hljs-literal">nil</span>
}
</code></pre>
<p>&#x5728;&#x5904;&#x7406;&#x5176;&#x4ED6;&#x8D44;&#x6E90;&#x65F6;&#xFF0C;&#x4E5F;&#x53EF;&#x4EE5;&#x91C7;&#x7528;defer&#x673A;&#x5236;&#xFF0C;&#x6BD4;&#x5982;&#x5BF9;&#x6587;&#x4EF6;&#x7684;&#x64CD;&#x4F5C;&#xFF1A;</p>
<p><u><i>io/ioutil</i></u></p>
<pre><code class="lang-Go"><span class="hljs-keyword">package</span> ioutil
<span class="hljs-keyword">func</span> ReadFile(filename <span class="hljs-keyword">string</span>) ([]<span class="hljs-keyword">byte</span>, error) {
    f, err := os.Open(filename)
    <span class="hljs-keyword">if</span> err != <span class="hljs-literal">nil</span> {
        <span class="hljs-keyword">return</span> <span class="hljs-literal">nil</span>, err
    }
    <span class="hljs-keyword">defer</span> f.Close()
    <span class="hljs-keyword">return</span> ReadAll(f)
}
</code></pre>
<p>&#x6216;&#x662F;&#x5904;&#x7406;&#x4E92;&#x65A5;&#x9501;&#xFF08;9.2&#x7AE0;&#xFF09;</p>
<pre><code class="lang-Go"><span class="hljs-keyword">var</span> mu sync.Mutex
<span class="hljs-keyword">var</span> m = <span class="hljs-built_in">make</span>(<span class="hljs-keyword">map</span>[<span class="hljs-keyword">string</span>]<span class="hljs-keyword">int</span>)
<span class="hljs-keyword">func</span> lookup(key <span class="hljs-keyword">string</span>) <span class="hljs-keyword">int</span> {
    mu.Lock()
    <span class="hljs-keyword">defer</span> mu.Unlock()
    <span class="hljs-keyword">return</span> m[key]
}
</code></pre>
<p>&#x8C03;&#x8BD5;&#x590D;&#x6742;&#x7A0B;&#x5E8F;&#x65F6;&#xFF0C;defer&#x673A;&#x5236;&#x4E5F;&#x5E38;&#x88AB;&#x7528;&#x4E8E;&#x8BB0;&#x5F55;&#x4F55;&#x65F6;&#x8FDB;&#x5165;&#x548C;&#x9000;&#x51FA;&#x51FD;&#x6570;&#x3002;&#x4E0B;&#x4F8B;&#x4E2D;&#x7684;bigSlowOperation&#x51FD;&#x6570;&#xFF0C;&#x76F4;&#x63A5;&#x8C03;&#x7528;trace&#x8BB0;&#x5F55;&#x51FD;&#x6570;&#x7684;&#x88AB;&#x8C03;&#x60C5;&#x51B5;&#x3002;bigSlowOperation&#x88AB;&#x8C03;&#x65F6;&#xFF0C;trace&#x4F1A;&#x8FD4;&#x56DE;&#x4E00;&#x4E2A;&#x51FD;&#x6570;&#x503C;&#xFF0C;&#x8BE5;&#x51FD;&#x6570;&#x503C;&#x4F1A;&#x5728;bigSlowOperation&#x9000;&#x51FA;&#x65F6;&#x88AB;&#x8C03;&#x7528;&#x3002;&#x901A;&#x8FC7;&#x8FD9;&#x79CD;&#x65B9;&#x5F0F;&#xFF0C; &#x6211;&#x4EEC;&#x53EF;&#x4EE5;&#x53EA;&#x901A;&#x8FC7;&#x4E00;&#x6761;&#x8BED;&#x53E5;&#x63A7;&#x5236;&#x51FD;&#x6570;&#x7684;&#x5165;&#x53E3;&#x548C;&#x6240;&#x6709;&#x7684;&#x51FA;&#x53E3;&#xFF0C;&#x751A;&#x81F3;&#x53EF;&#x4EE5;&#x8BB0;&#x5F55;&#x51FD;&#x6570;&#x7684;&#x8FD0;&#x884C;&#x65F6;&#x95F4;&#xFF0C;&#x5982;&#x4F8B;&#x5B50;&#x4E2D;&#x7684;start&#x3002;&#x9700;&#x8981;&#x6CE8;&#x610F;&#x4E00;&#x70B9;&#xFF1A;&#x4E0D;&#x8981;&#x5FD8;&#x8BB0;defer&#x8BED;&#x53E5;&#x540E;&#x7684;&#x5706;&#x62EC;&#x53F7;&#xFF0C;&#x5426;&#x5219;&#x672C;&#x8BE5;&#x5728;&#x8FDB;&#x5165;&#x65F6;&#x6267;&#x884C;&#x7684;&#x64CD;&#x4F5C;&#x4F1A;&#x5728;&#x9000;&#x51FA;&#x65F6;&#x6267;&#x884C;&#xFF0C;&#x800C;&#x672C;&#x8BE5;&#x5728;&#x9000;&#x51FA;&#x65F6;&#x6267;&#x884C;&#x7684;&#xFF0C;&#x6C38;&#x8FDC;&#x4E0D;&#x4F1A;&#x88AB;&#x6267;&#x884C;&#x3002;</p>
<p><u><i>gopl.io/ch5/trace</i></u></p>
<pre><code class="lang-Go"><span class="hljs-keyword">func</span> bigSlowOperation() {
    <span class="hljs-keyword">defer</span> trace(<span class="hljs-string">&quot;bigSlowOperation&quot;</span>)() <span class="hljs-comment">// don&apos;t forget the extra parentheses</span>
    <span class="hljs-comment">// ...lots of work&#x2026;</span>
    time.Sleep(<span class="hljs-number">10</span> * time.Second) <span class="hljs-comment">// simulate slow operation by sleeping</span>
}
<span class="hljs-keyword">func</span> trace(msg <span class="hljs-keyword">string</span>) <span class="hljs-keyword">func</span>() {
    start := time.Now()
    log.Printf(<span class="hljs-string">&quot;enter %s&quot;</span>, msg)
    <span class="hljs-keyword">return</span> <span class="hljs-keyword">func</span>() { 
        log.Printf(<span class="hljs-string">&quot;exit %s (%s)&quot;</span>, msg,time.Since(start)) 
    }
}
</code></pre>
<p>&#x6BCF;&#x4E00;&#x6B21;bigSlowOperation&#x88AB;&#x8C03;&#x7528;&#xFF0C;&#x7A0B;&#x5E8F;&#x90FD;&#x4F1A;&#x8BB0;&#x5F55;&#x51FD;&#x6570;&#x7684;&#x8FDB;&#x5165;&#xFF0C;&#x9000;&#x51FA;&#xFF0C;&#x6301;&#x7EED;&#x65F6;&#x95F4;&#x3002;&#xFF08;&#x6211;&#x4EEC;&#x7528;time.Sleep&#x6A21;&#x62DF;&#x4E00;&#x4E2A;&#x8017;&#x65F6;&#x7684;&#x64CD;&#x4F5C;&#xFF09;</p>
<pre><code>$ go build gopl.io/ch5/trace
$ ./trace
2015/11/18 09:53:26 enter bigSlowOperation
2015/11/18 09:53:36 exit bigSlowOperation (10.000589217s)
</code></pre><p>&#x6211;&#x4EEC;&#x77E5;&#x9053;&#xFF0C;defer&#x8BED;&#x53E5;&#x4E2D;&#x7684;&#x51FD;&#x6570;&#x4F1A;&#x5728;return&#x8BED;&#x53E5;&#x66F4;&#x65B0;&#x8FD4;&#x56DE;&#x503C;&#x53D8;&#x91CF;&#x540E;&#x518D;&#x6267;&#x884C;&#xFF0C;&#x53C8;&#x56E0;&#x4E3A;&#x5728;&#x51FD;&#x6570;&#x4E2D;&#x5B9A;&#x4E49;&#x7684;&#x533F;&#x540D;&#x51FD;&#x6570;&#x53EF;&#x4EE5;&#x8BBF;&#x95EE;&#x8BE5;&#x51FD;&#x6570;&#x5305;&#x62EC;&#x8FD4;&#x56DE;&#x503C;&#x53D8;&#x91CF;&#x5728;&#x5185;&#x7684;&#x6240;&#x6709;&#x53D8;&#x91CF;&#xFF0C;&#x6240;&#x4EE5;&#xFF0C;&#x5BF9;&#x533F;&#x540D;&#x51FD;&#x6570;&#x91C7;&#x7528;defer&#x673A;&#x5236;&#xFF0C;&#x53EF;&#x4EE5;&#x4F7F;&#x5176;&#x89C2;&#x5BDF;&#x51FD;&#x6570;&#x7684;&#x8FD4;&#x56DE;&#x503C;&#x3002;</p>
<p>&#x4EE5;double&#x51FD;&#x6570;&#x4E3A;&#x4F8B;&#xFF1A;</p>
<pre><code class="lang-Go"><span class="hljs-keyword">func</span> double(x <span class="hljs-keyword">int</span>) <span class="hljs-keyword">int</span> {
    <span class="hljs-keyword">return</span> x + x
}
</code></pre>
<p>&#x6211;&#x4EEC;&#x53EA;&#x9700;&#x8981;&#x9996;&#x5148;&#x547D;&#x540D;double&#x7684;&#x8FD4;&#x56DE;&#x503C;&#xFF0C;&#x518D;&#x589E;&#x52A0;defer&#x8BED;&#x53E5;&#xFF0C;&#x6211;&#x4EEC;&#x5C31;&#x53EF;&#x4EE5;&#x5728;double&#x6BCF;&#x6B21;&#x88AB;&#x8C03;&#x7528;&#x65F6;&#xFF0C;&#x8F93;&#x51FA;&#x53C2;&#x6570;&#x4EE5;&#x53CA;&#x8FD4;&#x56DE;&#x503C;&#x3002;</p>
<pre><code class="lang-Go"><span class="hljs-keyword">func</span> double(x <span class="hljs-keyword">int</span>) (result <span class="hljs-keyword">int</span>) {
    <span class="hljs-keyword">defer</span> <span class="hljs-keyword">func</span>() { fmt.Printf(<span class="hljs-string">&quot;double(%d) = %d\n&quot;</span>, x,result) }()
    <span class="hljs-keyword">return</span> x + x
}
_ = double(<span class="hljs-number">4</span>)
<span class="hljs-comment">// Output:</span>
<span class="hljs-comment">// &quot;double(4) = 8&quot;</span>
</code></pre>
<p>&#x53EF;&#x80FD;double&#x51FD;&#x6570;&#x8FC7;&#x4E8E;&#x7B80;&#x5355;&#xFF0C;&#x770B;&#x4E0D;&#x51FA;&#x8FD9;&#x4E2A;&#x5C0F;&#x6280;&#x5DE7;&#x7684;&#x4F5C;&#x7528;&#xFF0C;&#x4F46;&#x5BF9;&#x4E8E;&#x6709;&#x8BB8;&#x591A;return&#x8BED;&#x53E5;&#x7684;&#x51FD;&#x6570;&#x800C;&#x8A00;&#xFF0C;&#x8FD9;&#x4E2A;&#x6280;&#x5DE7;&#x5F88;&#x6709;&#x7528;&#x3002;</p>
<p>&#x88AB;&#x5EF6;&#x8FDF;&#x6267;&#x884C;&#x7684;&#x533F;&#x540D;&#x51FD;&#x6570;&#x751A;&#x81F3;&#x53EF;&#x4EE5;&#x4FEE;&#x6539;&#x51FD;&#x6570;&#x8FD4;&#x56DE;&#x7ED9;&#x8C03;&#x7528;&#x8005;&#x7684;&#x8FD4;&#x56DE;&#x503C;&#xFF1A;</p>
<pre><code class="lang-Go"><span class="hljs-keyword">func</span> triple(x <span class="hljs-keyword">int</span>) (result <span class="hljs-keyword">int</span>) {
    <span class="hljs-keyword">defer</span> <span class="hljs-keyword">func</span>() { result += x }()
    <span class="hljs-keyword">return</span> double(x)
}
fmt.Println(triple(<span class="hljs-number">4</span>)) <span class="hljs-comment">// &quot;12&quot;</span>
</code></pre>
<p>&#x5728;&#x5FAA;&#x73AF;&#x4F53;&#x4E2D;&#x7684;defer&#x8BED;&#x53E5;&#x9700;&#x8981;&#x7279;&#x522B;&#x6CE8;&#x610F;&#xFF0C;&#x56E0;&#x4E3A;&#x53EA;&#x6709;&#x5728;&#x51FD;&#x6570;&#x6267;&#x884C;&#x5B8C;&#x6BD5;&#x540E;&#xFF0C;&#x8FD9;&#x4E9B;&#x88AB;&#x5EF6;&#x8FDF;&#x7684;&#x51FD;&#x6570;&#x624D;&#x4F1A;&#x6267;&#x884C;&#x3002;&#x4E0B;&#x9762;&#x7684;&#x4EE3;&#x7801;&#x4F1A;&#x5BFC;&#x81F4;&#x7CFB;&#x7EDF;&#x7684;&#x6587;&#x4EF6;&#x63CF;&#x8FF0;&#x7B26;&#x8017;&#x5C3D;&#xFF0C;&#x56E0;&#x4E3A;&#x5728;&#x6240;&#x6709;&#x6587;&#x4EF6;&#x90FD;&#x88AB;&#x5904;&#x7406;&#x4E4B;&#x524D;&#xFF0C;&#x6CA1;&#x6709;&#x6587;&#x4EF6;&#x4F1A;&#x88AB;&#x5173;&#x95ED;&#x3002;</p>
<pre><code class="lang-Go"><span class="hljs-keyword">for</span> _, filename := <span class="hljs-keyword">range</span> filenames {
    f, err := os.Open(filename)
    <span class="hljs-keyword">if</span> err != <span class="hljs-literal">nil</span> {
        <span class="hljs-keyword">return</span> err
    }
    <span class="hljs-keyword">defer</span> f.Close() <span class="hljs-comment">// <span class="hljs-doctag">NOTE:</span> risky; could run out of file descriptors</span>
    <span class="hljs-comment">// ...process f&#x2026;</span>
}
</code></pre>
<p>&#x4E00;&#x79CD;&#x89E3;&#x51B3;&#x65B9;&#x6CD5;&#x662F;&#x5C06;&#x5FAA;&#x73AF;&#x4F53;&#x4E2D;&#x7684;defer&#x8BED;&#x53E5;&#x79FB;&#x81F3;&#x53E6;&#x5916;&#x4E00;&#x4E2A;&#x51FD;&#x6570;&#x3002;&#x5728;&#x6BCF;&#x6B21;&#x5FAA;&#x73AF;&#x65F6;&#xFF0C;&#x8C03;&#x7528;&#x8FD9;&#x4E2A;&#x51FD;&#x6570;&#x3002;</p>
<pre><code class="lang-Go"><span class="hljs-keyword">for</span> _, filename := <span class="hljs-keyword">range</span> filenames {
    <span class="hljs-keyword">if</span> err := doFile(filename); err != <span class="hljs-literal">nil</span> {
        <span class="hljs-keyword">return</span> err
    }
}
<span class="hljs-keyword">func</span> doFile(filename <span class="hljs-keyword">string</span>) error {
    f, err := os.Open(filename)
    <span class="hljs-keyword">if</span> err != <span class="hljs-literal">nil</span> {
        <span class="hljs-keyword">return</span> err
    }
    <span class="hljs-keyword">defer</span> f.Close()
    <span class="hljs-comment">// ...process f&#x2026;</span>
}
</code></pre>
<p>&#x4E0B;&#x9762;&#x7684;&#x4EE3;&#x7801;&#x662F;fetch&#xFF08;1.5&#x8282;&#xFF09;&#x7684;&#x6539;&#x8FDB;&#x7248;&#xFF0C;&#x6211;&#x4EEC;&#x5C06;http&#x54CD;&#x5E94;&#x4FE1;&#x606F;&#x5199;&#x5165;&#x672C;&#x5730;&#x6587;&#x4EF6;&#x800C;&#x4E0D;&#x662F;&#x4ECE;&#x6807;&#x51C6;&#x8F93;&#x51FA;&#x6D41;&#x8F93;&#x51FA;&#x3002;&#x6211;&#x4EEC;&#x901A;&#x8FC7;path.Base&#x63D0;&#x51FA;url&#x8DEF;&#x5F84;&#x7684;&#x6700;&#x540E;&#x4E00;&#x6BB5;&#x4F5C;&#x4E3A;&#x6587;&#x4EF6;&#x540D;&#x3002;</p>
<p><u><i>gopl.io/ch5/fetch</i></u></p>
<pre><code class="lang-Go"><span class="hljs-comment">// Fetch downloads the URL and returns the</span>
<span class="hljs-comment">// name and length of the local file.</span>
<span class="hljs-keyword">func</span> fetch(url <span class="hljs-keyword">string</span>) (filename <span class="hljs-keyword">string</span>, n <span class="hljs-keyword">int64</span>, err error) {
    resp, err := http.Get(url)
    <span class="hljs-keyword">if</span> err != <span class="hljs-literal">nil</span> {
        <span class="hljs-keyword">return</span> <span class="hljs-string">&quot;&quot;</span>, <span class="hljs-number">0</span>, err
    }
    <span class="hljs-keyword">defer</span> resp.Body.Close()
    local := path.Base(resp.Request.URL.Path)
    <span class="hljs-keyword">if</span> local == <span class="hljs-string">&quot;/&quot;</span> {
        local = <span class="hljs-string">&quot;index.html&quot;</span>
    }
    f, err := os.Create(local)
    <span class="hljs-keyword">if</span> err != <span class="hljs-literal">nil</span> {
        <span class="hljs-keyword">return</span> <span class="hljs-string">&quot;&quot;</span>, <span class="hljs-number">0</span>, err
    }
    n, err = io.Copy(f, resp.Body)
    <span class="hljs-comment">// Close file, but prefer error from Copy, if any.</span>
    <span class="hljs-keyword">if</span> closeErr := f.Close(); err == <span class="hljs-literal">nil</span> {
        err = closeErr
    }
    <span class="hljs-keyword">return</span> local, n, err
}
</code></pre>
<p>&#x5BF9;resp.Body.Close&#x5EF6;&#x8FDF;&#x8C03;&#x7528;&#x6211;&#x4EEC;&#x5DF2;&#x7ECF;&#x89C1;&#x8FC7;&#x4E86;&#xFF0C;&#x5728;&#x6B64;&#x4E0D;&#x505A;&#x89E3;&#x91CA;&#x3002;&#x4E0A;&#x4F8B;&#x4E2D;&#xFF0C;&#x901A;&#x8FC7;os.Create&#x6253;&#x5F00;&#x6587;&#x4EF6;&#x8FDB;&#x884C;&#x5199;&#x5165;&#xFF0C;&#x5728;&#x5173;&#x95ED;&#x6587;&#x4EF6;&#x65F6;&#xFF0C;&#x6211;&#x4EEC;&#x6CA1;&#x6709;&#x5BF9;f.close&#x91C7;&#x7528;defer&#x673A;&#x5236;&#xFF0C;&#x56E0;&#x4E3A;&#x8FD9;&#x4F1A;&#x4EA7;&#x751F;&#x4E00;&#x4E9B;&#x5FAE;&#x5999;&#x7684;&#x9519;&#x8BEF;&#x3002;&#x8BB8;&#x591A;&#x6587;&#x4EF6;&#x7CFB;&#x7EDF;&#xFF0C;&#x5C24;&#x5176;&#x662F;NFS&#xFF0C;&#x5199;&#x5165;&#x6587;&#x4EF6;&#x65F6;&#x53D1;&#x751F;&#x7684;&#x9519;&#x8BEF;&#x4F1A;&#x88AB;&#x5EF6;&#x8FDF;&#x5230;&#x6587;&#x4EF6;&#x5173;&#x95ED;&#x65F6;&#x53CD;&#x9988;&#x3002;&#x5982;&#x679C;&#x6CA1;&#x6709;&#x68C0;&#x67E5;&#x6587;&#x4EF6;&#x5173;&#x95ED;&#x65F6;&#x7684;&#x53CD;&#x9988;&#x4FE1;&#x606F;&#xFF0C;&#x53EF;&#x80FD;&#x4F1A;&#x5BFC;&#x81F4;&#x6570;&#x636E;&#x4E22;&#x5931;&#xFF0C;&#x800C;&#x6211;&#x4EEC;&#x8FD8;&#x8BEF;&#x4EE5;&#x4E3A;&#x5199;&#x5165;&#x64CD;&#x4F5C;&#x6210;&#x529F;&#x3002;&#x5982;&#x679C;io.Copy&#x548C;f.close&#x90FD;&#x5931;&#x8D25;&#x4E86;&#xFF0C;&#x6211;&#x4EEC;&#x503E;&#x5411;&#x4E8E;&#x5C06;io.Copy&#x7684;&#x9519;&#x8BEF;&#x4FE1;&#x606F;&#x53CD;&#x9988;&#x7ED9;&#x8C03;&#x7528;&#x8005;&#xFF0C;&#x56E0;&#x4E3A;&#x5B83;&#x5148;&#x4E8E;f.close&#x53D1;&#x751F;&#xFF0C;&#x66F4;&#x6709;&#x53EF;&#x80FD;&#x63A5;&#x8FD1;&#x95EE;&#x9898;&#x7684;&#x672C;&#x8D28;&#x3002;</p>
<p><strong>&#x7EC3;&#x4E60;5.18&#xFF1A;</strong>&#x4E0D;&#x4FEE;&#x6539;fetch&#x7684;&#x884C;&#x4E3A;&#xFF0C;&#x91CD;&#x5199;fetch&#x51FD;&#x6570;&#xFF0C;&#x8981;&#x6C42;&#x4F7F;&#x7528;defer&#x673A;&#x5236;&#x5173;&#x95ED;&#x6587;&#x4EF6;&#x3002;</p>

                                
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